The subject matter disclosed herein relates generally to gas turbine engine frames for supporting bearings and shafts, and, more specifically, to stiffening structures, such as rails, associated with gas turbine engine frame casings.
Gas turbine engines may include one or more rotor shafts supported by bearings which, in turn, may be supported by generally annular engine frames. An engine frame may include a generally annular casing spaced radially outwardly from an annular hub, with a plurality of circumferentially spaced apart struts extending therebetween. The struts may be integrally formed with the casing and hub in a common casting, for example, or may be suitably mechanically attached thereto. In either case, the engine frame may be configured to have suitable structural rigidity for supporting the rotor shaft and to minimize deflections of the rotor shaft during operation.
Engine frames may be configured to transmit loads from the internal rotor bearing support, through the hub, across the engine flowpath, such as by generally equally spaced struts, to flanges disposed on the case. Because the bearing load may be transferred into the case at local points, e.g., the strut ends, the design of the case may be important to the overall frame stiffness. Bending may occur in relatively thin annular case sections due to these point loads, which may introduce unwanted flexibility in the engine frame.
Thermal effects may play a role in the design of gas turbine engine frames, particularly to hot section applications. For example, a severe thermal gradient may develop between the hot casing, which may be at least partially exposed to engine core air on its inner surface, and relatively cool stiffener rings, which may be exposed to under-cowl air during engine operation. These gradients may cause thermal stresses that may lead to cracking and may sometimes require active heating of the reinforcing rings to avoid such distress.
The problem: For gas turbine engine frames having low numbers of struts, it may be difficult to provide a substantially direct load path on the casing between the struts while maintaining a substantially circular casing.